LatencyAwareHClientPool.java

/**
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package com.datastax.drivers.jdbc.pool.cassandra.connection;

import java.sql.SQLException;
import java.util.concurrent.LinkedBlockingDeque;
import java.util.concurrent.atomic.AtomicInteger;

import com.datastax.drivers.jdbc.pool.cassandra.jdbc.CassandraConnectionHandle;


/**
 * This class provides a queue function of latencies over CHCP, collecting all the latency information and calculates
 * the score (expensive operation).
 * 
 * @author Vijay Parthasarathy
 */
public class LatencyAwareHClientPool extends ConcurrentHClientPool {
  private static final AtomicInteger intervalupdates = new AtomicInteger(0);
  // Mostly static configuration this doesnt need to be configurable to the clients.
  private static final int UPDATES_PER_INTERVAL = 1000;
  private static final int WINDOW_QUEUE_SIZE = 100;
  private static final double SENTINEL_COMPARE = 0.768;
  private final LinkedBlockingDeque<Double> latencies;

  public LatencyAwareHClientPool(CassandraHost host) throws SQLException {
    super(host);
    latencies = new LinkedBlockingDeque<Double>(WINDOW_QUEUE_SIZE);
  }

  @Override
  public CassandraConnectionHandle borrowClient() throws SQLException {
    CassandraConnectionHandle conn = super.borrowClient();
    conn.startToUse();
    return conn;
  }

  @Override
  public void releaseClient(CassandraConnectionHandle client) throws SQLException {
    add(client.getSinceLastUsed());
    super.releaseClient(client);
  }

  void add(double i) {
    if (intervalupdates.intValue() >= UPDATES_PER_INTERVAL)
      return;
    if (!latencies.offer(i)) {
      latencies.remove();
      latencies.offer(i);
    }
    intervalupdates.getAndIncrement();
  }

  double score() {
    double log = 0d;
    if (latencies.size() > 0) {
      double probability = p(SENTINEL_COMPARE);
      log = (-1) * Math.log10(probability);
    }
    return log;
  }

  double p(double t) {
    double mean = mean();
    double exponent = (-1) * (t) / mean;
    return 1 - Math.pow(Math.E, exponent);
  }

  private double mean() {
    double total = 0;
    for (double d : latencies) {
      total += d;
    }
    return total / latencies.size();
  }

  public void resetIntervel() {
    intervalupdates.set(0);
  }

  public void clear() {
    latencies.clear();
    intervalupdates.set(0);
  }

  @Override
  public boolean equals(Object obj) {
    return ((LatencyAwareHClientPool) obj).getCassandraHost().equals(getCassandraHost());
  }
}